1. Field of Invention
The present invention relates to a photo-irradiation type heat treatment apparatus and specifically to a photo-irradiation type heat treatment apparatus provided with filament lamps as a light source for heating an article to be treated.
2. Description of Related Art
In various processes of semiconductor production, such as film formation, oxidation, azotization, film stabilization, crystallization and implanted ion activation, a method of heat treatment by irradiating light from a light source has recently been used. Specifically, the rapid thermal processing (RTP as used herein) used for rapidly increasing or decreasing the temperature of an article to be treated, such as semiconductor wafers, is very popular since it can enhance yield and quality.
In such a photo-irradiation type heat treatment apparatus, incandescent lamps are used as a light source, for example.
Incandescent lamps are provided with filaments inside a bulb made of a light transmissive material, radiate not less than 90% of input power and can heat an article to be treated without contacting it. Therefore, if they are used as a light source for heating glass substrates and semiconductor wafers, incandescent lamps can rapidly increase or decrease the temperature of an article as compared with a resistance heating method. More specifically, they can increase the temperature of an article up to at least 1000° C. within several seconds to several tens of seconds and the article rapidly cools after irradiation is stopped.
As shown in FIG. 11, a conventional photo-irradiation type heat treatment apparatus is provided with a chamber 41 formed by dividing its inner space into an upper space (lamp unit accommodating space S1) and a lower space (heat treatment space S2) by a plate-like light transmitting window 42. The lamp unit accommodating space S1 comprises a lamp unit 50 constituted of multiple filament lamps 51 arranged in such a way as to constitute a planar array light source and a reflecting mirror 52 is arranged above the filament lamps 51 in such a way as to reflect light emitted upward from the filament lamps 51 back toward an article W to be treated, whereby light emitted from each filament lamp 51 is directly, or after being reflected by the reflecting mirror 52, delivered to the article W placed in the heat treatment space S2 through the light transmitting window 42. As a result, the article W is heated to a specified temperature.
As semiconductor elements have recently been reduced to micron size and steadily advanced in performance, the demand for uniform temperature distribution of an article to be treated has increased more than ever.
As an example, when heating an article to 1100° C., it is required to heat the article in such a way that the temperature error is within the range of +/−1° C. In order to meet this requirement, one method is to shorten the distance between a filament lamp and the article to be treated so that the irradiance distribution can become uniform on the surface of the article.
However, even if a photo-irradiation type heat treatment apparatus is constructed in such a way that the distance between a filament lamp and the article becomes small, it has been found to be difficult to make the temperature error small on the surface of the article in practice. This is for the following reason.
In the photo-irradiation type heat treatment apparatus, filament lamps are cooled by blowing a large amount of cooling air on the surface of the filament lamps in order to prevent the surface of the filament lamps from becoming too hot. Accordingly, a light transmitting window placed on the light exit side of the filament lamps is also cooled together with the filament lamps. Specifically, in the conventional photo-irradiation type heat treatment apparatus as shown in FIG. 11, cooling air (as shown by a hollow arrow in FIG. 11) is introduced into the lamp unit accommodating space S1 from a cooling air supply passage 45 located on one side of filament lamps 51 in the array direction flows along a light transmitting window 42 and is discharged from a discharge air passage 46, whereby the temperature of cooling air is high on the downwind side and low on the upwind side. As a result, the light transmitting window 42 shows a temperature distribution dependent on the direction of passing cooling air (i.e., a temperature distribution in which temperature is low on the upwind side and high on the downwind side).
Moreover, since the light transmitting window is usually made larger than the effective irradiation area of the lamps, light emitted from the lamps irradiates over a larger area at the peripheral portions of the light transmitting window. As a result, a temperature distribution is such that temperature is high at the central area and low at the peripheral portions. In consequence, a temperature distribution of the light transmitting window has a distorted form by a combination of a temperature distribution dependent on the passing direction of cooling air and a temperature distribution dependent on radiation intensity. Moreover, various changes under the influence of the volume of cooling air and the fluctuation of the illuminance of lamps also affects the temperature distribution.
Thus, in the conventional photo-irradiation type heat treatment apparatus in which a light transmitting window and an article to be treated are placed close to each other, it is difficult to control the temperature of the light transmitting window, which is believed to lead to a lack of uniformity in the temperature distribution of the article under the adverse influence of a temperature distribution of the light transmitting window. On the other hand, another photo-irradiation type heat treatment apparatus has been disclosed, wherein an opening is provided on a reflecting mirror placed above multiple filament lamps and cooling air is flowed in through this opening in the direction of the normal line of the virtual plane formed by the filament lamps (See Japanese Unexamined Patent Publication No. 2000-306857 and corresponding U.S. Pat. No. 6,414,279).
It is said that the problems in obtaining a uniform temperature distribution of an article to be treated under the influence of a temperature distribution of the light transmitting window can be solved even if the light transmitting window and the article are placed close to each other. However, the simple provision of an opening on a reflecting mirror causes a problem that the radiation intensity of an article to be treated declines because the area of the effective reflecting surface of the reflecting mirror decreases to the point that the original function of the reflecting mirror is impaired.